Low pressure fluorescent lamps cannot normally be operated in parallel electrical paths because the breakdown/starting voltage is larger than the operating voltage, and the lamps cannot be made such that a discharge is initiated in all lamps simultaneously. After one lamp breaks down, the voltage applied to all lamps drops significantly and is not large enough to break down the other lamps. Even if a discharge could be initiated in all lamps, discharge lamps driven at low frequencies or with DC cannot be operated in parallel without ballasting because low pressure discharges have a negative voltage/current (V/I) characteristic. The negative V/I characteristic means that as the lamp current increases, the lamp voltage decreases. Thus, when several lamps are connected in parallel and all are ignited, one lamp normally operates at a higher current and lower discharge voltage than the others. This lamp will cause all the other lamps to be extinguished.
Compact fluorescent lamps are considerably more efficient than incandescent lamps, but they are limited in total lumen output. For example, a compact fluorescent lamp about the same size as a 100 watt incandescent lamp provides about 900 lumens, which is sufficient to replace only a 60 watt incandescent lamp. The total amount of light emitted per unit volume can be increased considerably by reducing the diameter of the discharge. With a reduced diameter, the electron temperature increases and the maximum light emitted per unit volume increases. To achieve a higher lumen output from a smaller diameter fluorescent lamp, however, the discharge length must be increased to keep the discharge volume constant. The result is a long, small diameter discharge which requires an intricate pattern of multiple bends to provide a compact lamp. Although technically feasible, this lamp is impractical and expensive because it is fragile and is virtually impossible to mass produce.
Current compact fluorescent lamps use a twin tube or double twin tube architecture. Twin tube fluorescent lamps typically include a pair of straight tubes that are interconnected at or near one end to form a generally U-shaped tube. These lamps are an improvement over a single large diameter tube, but even the double twin tube lamp is limited to four tubes driven in series. If eight tubes are needed, two double twin tubes may employed, but this requires two ballasts and is expensive as well as bulky and impractical.
Electrodeless fluorescent light sources utilizing inductive coupling have been disclosed in various U.S. patents. A closed loop magnetic core transformer, contained within a reentrant cavity in the lamp envelope, induces a discharge in an electrodeless fluorescent lamp in U.S. Pat. No. 4,005,330 issued Jan. 25, 1977 to Glascock et al. The discharge is induced by a magnetic core coil within the envelope of an electrodeless fluorescent lamp in the light source disclosed in U.S. Pat. No. 4,017,764 issued Apr. 12, 1977 to Anderson. In both of the above mentioned patents, the operating frequency is limited to about 50 kHz because of the lossy nature of magnetic materials at high frequency. An electrodeless fluorescent light source utilizing an air core coil for inductive coupling at a frequency of about 4 MHz is disclosed in U.S. Pat. No. 4,010,400 issued Mar. 1, 1977 to Hollister. However, such a light source has a tendency to radiate power at the frequency of operation and exhibits nonuniform plasma excitation.
An electrodeless fluorescent light source, utilizing frequencies in the 100 MHz to 300 GHz range, is disclosed by Haugsjaa et al in U.S. Pat. No. 4,189,661 issued Feb. 19, 1980. High frequency power, typically at 915 MHz, is coupled to an ultraviolet-producing low pressure discharge in a phosphor-coated electrodeless lamp which acts as a termination within a termination fixture.
A compact fluorescent light source wherein high frequency power is capacitively coupled to a low pressure discharge is disclosed in U.S. Pat. No. 4,266,167 issued May 5, 1981 to Proud et al. The lamp envelope has an outer shape similar to that of an incandescent lamp. An outer conductor, typically a conductive mesh, is disposed on the outer surface of the lamp envelope, and an inner conductor is disposed in a reentrant cavity in the lamp envelope. Frequencies in the range of 10 MHz to 10 GHz are suggested. An electrodeless discharge tube wherein high frequency energy is coupled to a discharge through external electrodes is disclosed in U.S. Pat. No. 4,798,997 issued Jan. 17, 1989 to Egami et al. Another electrodeless fluorescent light source which is energized by a high frequency power source is disclosed in U.S. Pat. No. 4,427,923 issued Jan. 24, 1984 to Proud et al.
It is a general object of the present invention to provide improved fluorescent light sources.
It is another object of the present invention to provide a fluorescent light source wherein multiple fluorescent lamp tubes are electrically connected in parallel to a single RF source.
It is a further object of the present invention to provide a compact fluorescent lamp having high lumen output.
It is yet another object of the present invention to provide fluorescent light sources which are low in cost and which are easy to manufacture.